Earth and Planetary Sciences ETDs

Dean Eppler

Publication Date

5-1976

Abstract

The San Antonio Mountain area is late Tertiary to early Quaternary volcanic terrane on the western edge of the San Luis Basin in the Rio Grande depression.

Calc-alkalic rocks are interlayered with flows of the Servilletta Basalt (Butler, 1946) and consist of a series of early basaltic rocks (Wissmath Craters basalt, basaltic andesites of Los Cerritos de la Cruz, Red Hill, Malette Creek), intermediate age basaltic andesite, andesite and latite of the San Antonio Mountain Volcanic Complex and late basalt of the Piñabetoso Peaks.

Data suggests that the volcanic rocks of the San Antonio Mountain area evolved from two magmas. One originated at shallow depth in a mantle bulge beneath the Rio Grande Depression and was erupted as the Servilletta Basalt (Lipman, 1969). This until is olivine tholeiite and is widespread throughout the San Luis Basin. The second magma was calc-alkalic. Three separate magmas were segregated from the parent magma and emplaced beneath the field area at different times and location. One of the magmas was erupted as the early basalt and basaltic andesites. Evolution of the second magma formed the early andesite and latite of the San Antonio Mountain Volcanic Complex. The third magma was erupted as the late basaltic andesite and andesite of San Antonio Mountain. The chemical evolution of these magmas may have been the result of fractionation or crustal contamination. The basalt of Pinabetoso Peaks appears to be only slightly differentiated and may have been derived directly from the original parent magma or from a different source magma.

The Volcanos of the field area can be classified in four groups, based on shape and internal structure. Tholeiite shield volcanos (Wissmath Craters) were constructed of thin lava flows dipping at low angles away from the vent. Cinder and lava cones (north and south Pinabetoso Peaks) consist of cinders and spatter which have been intruded by later lavas. The intrusions dip toward a central vent which is also probably the source of the pyroclastics. Lava cones (north and south Cerritos de la Cruz, Red Hill) consist of lava flows which have piled up around a central vent and have steep, quaaquaversal dips. Some lava cones, however, have flows which dip toward the vent as well as away. These volcanos are often breached by later explosions. Flow domes (San Antonio Mountain) are constructed of thick lava flows which have been erupted from a central vent and have piled up around the vent to form a steep, domical structure. These flows which have steep, quaquaversal dips. Pyroclastic activity also forms hills of agglomerate around the central vent.

Correlation of external shape with bulk chemistry indicates that for large structures, such as shields and flow domes, composition of the lava flows is the major factor in determining the shape of the volcano. With increasing silica content, flows are thicker, more viscous and travel for shorter distances from the vent, forming steep sided structures. For these volcanos eruptive history does not appear to greatly affect the final shape of the volcano. In smaller structures, such as lava cones, eruptive history and vent conditions during eruption appear to be more significant than bulk chemistry in determining the shape of the volcano.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Wolfgang Eugene Elston

Second Committee Member

Lee A. Woodward

Third Committee Member

Albert Masakiyo Kudo

Fourth Committee Member

Illegible

Language

English

Document Type

Thesis

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